In examination of water pollution, analysis of food and the like, analysis of 7 kinds of ions, i.e., fluoride ion (F−), chloride ion (Cl−), nitrite ion (NO2−), bromide ion (Br−), nitrate ion (NO3−), sulfate ion (SO42−), and phosphate ion (PO43−) are important and called “7 standard inorganic anions”. Recently, in the analysis of inorganic anions including the 7 standard inorganic anions, ion chromatography is being used as an efficient and high-precision/high-sensitive means for analyzing them.
In the ion chromatography, a sample containing an ion seed is injected into an ion exchange column while feeding an eluent into the column and the ions (kind, amount) separated and eluted from the column with a time gap due to the difference in the retention time are detected by a high-sensitivity detector such as electrical conductivity detector. The ion chromatography includes “a suppressor method” using a suppressor and “a non-suppressor method” using no suppressor.
In the non-suppressor method, organic acids having low conductivity such as phthalic acid, p-hydroxybenzoic acid, and trimesic acid are used and the anions are detected directly by a conductivity detector immediately after they were separated by a separation column. The conductivity detector is low in background so that high-sensitivity analysis is possible. The pH of the eluent is not particularly limited and various separation conditions can be selected.
Generally used packing (hereinafter, also referred to as “anion exchanger”) of a column for a non-suppressor method includes chemical bond type ion exchangers comprising copolymer particles of a vinyl ester- or vinyl ether-containing monomer having a hydroxyl group and a vinyl ester- or vinyl ether-crosslinking monomer.
The column for a non-suppressor method that has been being conventionally used is packed with the above packing. In order to make the column high-powered, packing must be downsized than ever.
However, the above packing has insufficient mechanical strength and it has not been easy to reduce its particle diameter.
Under the circumstances, a first object of the present invention is to provide a porous polymer particles having high mechanical strength useful for anion analysis liquid chromatography for a suppressor method and a non-suppressor method, and anion exchanger comprising the particles and a producing method thereof.
On the other hand, in the ion chromatography by a suppressor method, a suppressor, which is an apparatus for substituting the cations in the liquid by hydrogen ions is used. As shown in FIG. 1, the suppressor is connected to between a separation column and a detector and functions to decrease the background and increase the sensitivity of measurement when detecting ions using an electrical conductivity detector.
In “the suppressor method”, a mixed solution of sodium carbonate and sodium hydrogencarbonate, a borate buffer, an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution or the like is passed through as the eluent to separate the sample ion in the separation column and thereafter the ion separated is detected by an electrical conductivity detector through a suppressor. The electrical conductivity measured by the detector includes the electrical conductivity of the eluent itself as the background and is comprehended as an superposed signal that comprises contributions of ion species in the sample. The suppressor converts a salt or base in the eluent to an acid or the like having a lower degree of dissociation to thereby decrease the background electrical conductivity and as a result improves the measurement sensitivity of signals attributable to the ion species of the sample.
Because of its high sensitivity, the suppressor method is indispensable to the management of pure water, chemicals and the like for use in the semiconductor art, though a dedicated apparatus is necessary and the profitability is inferior as compared with the non-suppressor method.
The anion exchanger used in the suppressor method is required to be stable at a relatively high pH (a pH on the order of from 9 to 12) and have a capability of successfully separating the objective anion. Specifically, the anion exchanger includes so-called pellicular-type ion exchanger obtained by coating an anion exchangeable latex on a sulfonated polystyrene substrate, and a porous chemical bond-type ion exchanger obtained by chemical-bonding a quaternary ammonium group to a porous polymer substrate, particularly an acrylate polymer substrate. Among these, the former ion exchanger has excellent alkali resistance and is being used on the greatest occasions. In the case of the latter ion exchanger, commercially available products at present are not sufficiently high in the alkali resistance and these are now scarcely used in the suppressor method but are used in many cases in the non-suppressor method mainly using an acidic eluent.
The pellicular-type ion exchanger restricts the migration of ions only to the surface of the packing and does not allow their entering into pores. Therefore, this ion exchanger is advantageous in that (1) the diffusion is almost prevented from occurring and (2) the ion does not interfere with the substrate. However, this ion exchanger is disadvantageous in that the usable surface area of the packing is limited in view of the structure and the column efficiency disadvantageously has a bound. For elevating the column efficiency of the pellicular-type ion exchanger, it is necessary to increase the column length or to reduce the particle size of the packing. However, the column used at present already has a large length of 250 mm and a more increase in the column length is not practical. With respect to the reduction in the particle size, even the packing having a particle size of about 5 μm, which is commonly used in the high-performance liquid chromatography, is very difficult to manufacture due to the limitation in view of the structure. Therefore, the pellicular-type ion exchanger cannot satisfy the requirement to have higher performance than the current theoretical plate number of 6,000 plates/column.
On the other hand, the porous chemical bond-type ion exchanger is excellent in the effective surface area of the packing because the ion migrates into pores to undertake the ion exchange. Therefore, this ion exchanger has a possibility of achieving higher performance. Although the porous chemical bond-type ion exchanger has a problem in that the peak is broadened due to the interference between the objective ion and the substrate, this broadening of the peak may be reduced by the manufacturing method described in JP-A-62-79356 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), where a spacer is introduced into the substrate and thereafter an ion exchange group is introduced. However, the porous chemical bond-type ion exchangers known at present cannot show sufficiently high alkali resistance at a pH of 9 or more and also lack the strength large enough to reduce the particle size and thereby achieve high performance.
In the analysis of anion using ion chromatography, the carbon dioxide gas contained in the sample is also detected as hydrogencarbonate ion. Therefore, in order to reduce the effect thereof to a minimum, a mixed aqueous solution of sodium carbonate and sodium hydrogencarbonate is widely used. However, in the analysis of trace components of a ppb level or less, hydrogencarbonate ion appears as a broad peak while overlapping with the peak of the objective ion to be analyzed and hence the objective ion is difficult to separate and quantitate. This broad peak appears as a recession or a projection from the base line depending on the concentration of the carbonate buffer solution used as the eluent and this recession or projection is called a carbonate dip or carbonate system peak (hereinafter referred to as a “carbonate dip”).
The anion exchanger heretofore used mainly for the suppressor system column includes the above-described two kinds of ion exchangers, namely, a pellicular-type ion exchanger obtained by coating an anion exchangeable latex on a sulfonated polystyrene substrate and a porous chemical bond-type ion exchanger obtained by introducing an anion exchange group into a polyvinyl alcohol substrate. These two kinds of ion exchangers greatly differ in the hydrogencarbonate ion-holding ability because of the difference in the hydrophilicity of the substrate. However, whichever ion exchanger is used, a technique for manufacturing a column capable of controlling the appearance position of carbonate dip on the chromatogram has not yet been established. Thus, a problem arises at the time of microanalysis.
In order to avoid this problem, a manufacturing method of producing a copolymer of a styrene-type monomer having high hydrophobicity and an alcohol-type monomer having high hydrophilicity with controlling the ratio of each monomer mixed, thereby controlling the appearance position of carbonate dip, has been proposed as disclosed in JP-A-9-124729. However, the ion exchanger produced by this method shows a nitrate ion peak having a very bad shape with extreme tailing. The peak shape is evaluated based on the value (Fas) obtained by drawing a perpendicular line from the top of the chromatogram peak obtained, determining the horizontal widths in the right and in the left, respectively, of the peak from the perpendicular line at 10% of the peak height, and dividing the right width by the left width. In general, the chromatogram peak is almost bilaterally symmetric and accordingly, the Fas is in the vicinity of 1.0. However, in the case of the ion exchanger obtained by the method of the above-described publication, the Fas is 5.0 and by far larger. Thus, the quantitation of nitrate ion is not practical and the ion seeds eluting after the nitrate ion are also adversely affected. Therefore, this ion exchanger is not suitable for the microanalysis.
In the anion analysis using ion chromatography, it is ideal that the 7 standard inorganic anions be separated with a good balance in an analysis time as short as possible. However, fluoride ion is difficult to be held by the anion exchanger in the separation column and passes fast through the column. As a result, separation between the signal peak by the fluoride ion and water dip (negative peak appearing due to dilution of eluent by the injection of a sample) is insufficient so that the precision of quantitation tends to be deteriorated.
It would be advisable to use an eluent having a weak eluting power in order to increase with time in of fluoride ion. In this case, the elution time for divalent or more anions (sulfate ion and phosphate ion) is very long to make the analysis time redundant. In particular, where the eluent is alkaline, this problem is severe. For this reason, to simultaneously analyze fluoride ion and a divalent or more anion, the analysis conditions must be sophisticated.
Hence, a method for avoiding the above problems by optimization of the composition of eluent is being studied. For example, in the non-suppressor method, a method is proposed in which boric acid is added to the moving phase, which is weakly acidic and the boric acid and fluoride ion are allowed to selectively react to form an anionic compound, thereby increasing the holding ability as disclosed in JP-B-7-37972 (the term “JP-B” as used herein means an “examined Japanese patent publication”) On the other hand, in the suppressor method, where a mixed solution of sodium carbonate and sodium hydrogencarbonate is used as an eluent, it has been known that varying the ratio of components can increase the holding ability of fluoride ion. Furthermore, a suppressor method in which a boric acid salt compound is added has been proposed as disclosed in JP-A-2000-180429. Thus, where the eluent can be constituted by a plurality of components, the above problems can be coped with by varying the composition thereof.
However, hydroxide base eluents such as aqueous sodium hydroxide solution and aqueous potassium hydroxide solution used as the eluent for a suppressor method are usually composed of a single component, so that the problems attributable to the eluent cannot be avoided. Therefore, in the suppressor method using such an alkaline eluent, a special technique must be used in order to make compatible the increase in the holding ability of fluoride ion and shortening of elution time for divalent or more anions (in particular phosphate ion among the 7 standard inorganic anions).
There have conventionally been practiced two methods. One is a gradient analysis method in which a concentration gradient is made in the eluent and another is a method in which the ion exchange capacity of the ion exchanger to be packed in the column is set to a greater value and a high concentration eluent as high as about 40 mM is used.
However, the first method is disadvantageous in that in order to impart the concentration gradient, it is necessary to provide at least two kinds of liquids having different concentrations and use an apparatus and operation for sucking and mixing them by means of two pumps and that a stabilization time during which the concentration of the eluent is returned to the original one for each measurement. The second method is disadvantageous in that because of the high concentration of eluent, the suppressor apparatus using an ion exchange membrane of a continuous regeneration type widely used at present requires application of high voltage for electrodialysis, which shortens the lifetime of suppressor.
When analyzing city water by a suppressor method using a hydroxide base eluent, not only improvement in the ability of holding fluoride ion and shortening of elution time for phosphate ion must be balanced but also sufficient separation of chloride ion and nitrite ion must be achieved at the same time. This is because in the analysis of city water, it is necessary to analyze nitrite ion on the order of several ppb in the presence of chloride ion on the order of several tens ppm. In the conventional column used for hydroxide base eluents, separation of chloride ion and nitrite ion is insufficient or carbonate ion is eluted during the separation if the separation is sufficient so that the analysis of trace amounts of nitrite ion is difficult to achieve at the same time.
Under the circumstances, a second object of the present invention is to provide a porous polymer particle for suppressor system ion chromatography, an anion exchanger comprising the particle, and a producing method thereof, where the particle is stable at high pH and favored with high-strength, which is capable of restricting elution time for phosphate ion to a short time on the order of from ten and several minutes to 30 minutes using a low concentration (for example, 20 mM or less) eluent without using gradient analysis, i.e., under isocratic conditions where the concentration is constant, sufficiently separating fluoride ion, which is difficult to hold, from water dip, and sufficiently separating chloride ion and nitrite ion to control the appearance position of carbonate dip not to overlap with the position of other ion peaks.
Further, a third object of the present invention is to provide a porous polymer particle in which a hydroxyl group or a carboxyl group is coexistent, an anion exchanger comprising the particle, and a producing method thereof, where in order to control the position of elution of each ion when anions are measured by a column for suppressor system or non-suppressor system ion analyzing chromatography for anion analysis, the particle is produced by introducing a quaternary ammonium group into a substrate which contains an ester bond as packing and treating with an alkaline solution to generate a hydroxyl group or a carboxyl group that can have negative charges repelling with the anions.